The free-theorems package

The free-theorems library allows to automatically generate free
theorems from Haskell type expressions. It supports nearly all
Haskell 98 types except of type constructor classes, and in
addition it can also handle higher-rank functions. Free theorems
are generated for three different sublanguages of Haskell, a
basic one corresponding to the polymorphic lambda-calculus of
Girard-Reynolds, an extension of that allowing for recursion and
errors, and finally a sublanguage additionally allowing seq.
In the last two sublanguages, also inequational free theorems
may be derived in addition to classical equational results.